The wind turbine market is changing fast nowadays. There is a continuing demand for larger wind turbines being able to generate a higher number of megawatts of electricity, also referred to as multi-megawatt wind turbines. At the same time the requirements for reduction of size and weight of the turbines and their components become more and more important.
In wind turbines, typically a wind turbine rotor drives a low speed shaft of a gear transmission unit or gearbox, which transforms torque and speed of the rotor to the required torque and speed of an electrical generator. The increasing demand for multi-megawatt wind turbines puts a challenging pressure on new designs of components for such wind turbines. This is because weight and cost of the wind turbine are to be kept as low as possible or at least within acceptable ranges, while at the same time it has to be made sure that the components can withstand high rotor loads being generated during operation of the wind turbine.
In existing turbine designs, two main categories can be distinguished. A first category comprises turbines with nacelle main frame structures that are strong enough on their own to handle all the loads that come from the rotor. An example of a turbine design belonging to this first category is turbine in which the nacelle main frame structure comprises a conventional bedplate design with two main bearings holding the main shaft. This, however, results in a heavy nacelle main frame, on which then a heavy drive train comprising, for example, a gearbox and/or a generator, is mounted. The reason why the drive train is made relatively heavy is that it has to be able to take full torque load. This results in a very heavy top mass of the wind turbine.
Design concepts in this first category do have a benefit of easy exchangeability of components of the drive train, but a major disadvantage of these design concepts is that the drive train itself is very heavy and does only partly participates in transferring rotor loads to the tower of the wind turbine. As a consequence a more robust nacelle main frame structure is required, which consequently results in a higher total weight of the nacelle main frame structure.
A second category comprises turbines with integrated nacelle main frame structure designs, in which the nacelle main frame structure and drive train form one unit. The benefit of such designs is that structural components are used in a more optimal way for transferring rotor loads to the tower. This is a more optimal use of the load carrying components and therefore the weight of the nacelle main frame structure can be reduced with respect to designs in the first category. However, these integrated designs make it more difficult for components to be serviced or replaced, which results in lower exchangeability possibilities and higher costs for service and maintenance activities.
It is clear from the above that for current existing turbine designs a compromise has to be found between optimal exchangeability possibilities of wind turbine components and optimal weight of the nacelle main frame structure.